An image displayed on a cathode ray tube (CRT) may suffer from imperfections or distortions such as defocusing or nonlinearity that is incident to the scanning of the beam on the CRT. Such imperfections or distortions occur because the distance from the electron gun of the CRT to the faceplate varies markedly as the beam is deflected, for example, in the horizontal direction. Typically, reducing the defocusing that occurs as the beam is deflected in the horizontal direction may be obtained by developing a dynamic focus voltage having a parabolic voltage component at the horizontal rate and applying the dynamic focus voltage to a focus electrode of the CRT for dynamically varying the focus voltage.
In projection television receiver (PTV), the blue phosphor of the screen of the CRT has a significantly lower persistency than the green or red phosphor. To compensate for the persistency difference, the instantaneous brightness of the blue phosphor is made significantly higher than that of the green or red phosphor. Because of the higher instantaneous brightness, the blue phosphor saturates at high video drive levels. Thus, light output does not increase in blue as video drive increases. In contrast, light output in the red and green phosphors continues to increase as video drive increases. The result is yellow in high light areas formed on the screen.
Obtaining high brightness of the picture for a given CRT may be desirable. Therefore, blue focus is typically set to be slightly defocussed for increasing the blue beam spot size. The increase in the beam spot size increases the saturation threshold and allows brighter blue and increased linear blue drive range. Slight blue defocusing is acceptable. This is so because the human eye cannot focus on blue and red at the same time and most picture detail is conveyed in red and green.
Beam current changes resulting from video loading cause variations of the ultor voltage. Despite having blue defocused in most areas of the picture, in some areas of the picture focussing may remain optimal, when the focus voltage does not track the ultor voltage.
It may be desirable to maintain consistent defocussing over the entire picture. Therefore, it may be desirable to make the focus voltage track changes in ultor voltage by maintaining a constant ratio of focus voltage to ultor voltage of, for example, 1/3. If this ratio tracking requirement is not met everywhere in the picture, there will be portions of the picture where the ultor voltage changes with respect to the blue focus voltage will cause undesirable ideal blue focussing. This will cause blue phosphor saturation and a lack of desired blue brightness may occur. The areas affected will appear as undesirable yellow horizontal stripes in white areas.
It may be desirable to provide correct focus voltage-to-ultor voltage ratio tracking so that the blue picture can be uniformly defocused for obtaining increased brightness with minimum yellow in the white areas.
In one prior art, focus voltage-to-ultor voltage ratio tracking was obtained by applying a portion of the ultor voltage to an input terminal of a dynamic focus amplifier such that a sample of the ultor voltage is summed in the amplifier with dynamic focus signals. A composite signal developed at an output terminal of the amplifier is then coupled to the focus electrode of each CRT of the PTV receiver through dynamic focus coupling capacitors